Tapered Wave Transducer

Transducers and acoustical lenses are commonly used for ultrasonic imaging. Standard transducers comprise a transducing element which is commonly made of a flat piezoelectric material and metallic electrodes on both sides. The lens is located in front of the transducer and is used to focus the mechanical waves produced by the transducer. To operate the transducer, an electrical signal is applied to the electrodes. In response, the transducing element produces mechanical waves. The mechanical waves travel through the lens, strike the object being imaged, and produce echoes. Some of these echoes return through the lens and strike the transducing element. In response to the mechanical waves, the transducing element produces an electrical signal on the electrodes. This electrical signal is used to generate an image.

The lens is used to focus the mechanical waves at a target in the object. To produce high resolution, the waves must be focussed to a narrow, concentrated beam. However, standard transducers produce uniform intensity, plane waves, and these waves, when focussed by a standard converging lens, produce "side lobes" in addition to the desired narrow beam or "main lobe". The intensity pattern is similar to a Bessel function waveform. These sidelobes indicate that mechanical waves are striking regions outside of the target, and thus, generating unwanted echoes. Some of these echoes proceed back to the transducer and are summed with those echoes produced by the main lobe mechanical waves. Standard imaging processors cannot distinguish the unwanted echoes from those of the main lobe. Thus, the echoes from the side lobes distort the image.

The invention is a transducer comprising a transducing element such as a piezoelectric disc, and electrodes. The electrodes can be made of a resistive material, or the electrodes can have an axial region made of a metallic, highly conducting material and a "washer-shaped" peripheral region made of a resistive material. The electrodes attach to two sides of the transducing element. A standard, converging, acoustical lens can be used to focus the mechanical waves produced by the transducer. The purpose of the resistive material in the electrodes is to attenuate electrical signals which are applied to and received by the peripheral region of the transducing element. This technique will attenuate the sidelobes of the transmitted beam and will accentuate the contribution of echoes originating from the region of the main lobe (focal zone).

FIG. 1 is a top view of the first embodiment of the invention and shows a disc-shaped electrode and the tops of the wires that attach to each electrode.

FIG. 2 is a cross-section of first embodiment of the invention, but not drawn to scale. The wire attaching to the bottom electrode fits through a hole in the transducing element and top electrode.

FIG. 3 shows the first embodiment transmitting mechanical waves through a converging, acoustical lens, and the acoustical beam of the main lobe. The transducer emits waves whose magnitude decreases with distance from the axis.

FIG. 4 is a top view of the third embodiment.

FIG. 5 shows a cross-section of the third embodiment and its transmitted mechanical waves. The length of the arrows roughly approximates the magnitude of the mechanical waves transmitted from the different regions.

Standard electrodes for transducing elements are made of metal, usually gold or silver. The purpose of using metal is to provide high conductivity and thus, to allow the application of an almost equal electrical signal to all areas of the transducing element. The metal is usually sputtered onto the transducing element. The first embodiment of the invention is shown in FIGS. 1 and 2. The electrodes 2 and 4 are disc-shaped, and made of a resistive material. The resistive material may comprise carbon. It may be mixed into gold or silver or another metal, and sputtered onto the transducing element. The resistive material may comprise any other known resistive material, including naturally resistive metals or alloys. The resistive materials can be attached to the transducing elements 6 in any known manner. The transducing element can be of any standard variety including standard piezoelectrics.

The transducer also comprises wires 8 and 10 attached near its axis. These wires bring the electrical signals to the electrodes. Since these wires attach near the axis of the electrodes, this region and the underlying region of the transducing element receive the largest electrical signals. Regions of the electrodes located away from the axis receive smaller electrical signals because some of the energy of those signals is dissipated in the resistive material of the electrodes. Thus, regions of the transducing element located away from the axis produce mechanical waves having less magnitude than the waves produced by the region about the axis. As a result, the magnitude of mechanical waves produced by the transducing element steadily decreases as the distance from the center increases, as shown by the length of the arrows 12 in FIG. 3. Note that the arrows are not drawn exactly to scale.

These "tapered" mechanical waves, when focussed by a converging 14 upon target 16, produce a beam with an intense center lobe 18 and attenuated side lobes.

The echoes which strike the transducing element produce electrical signals on the electrodes. The electrical signals located on the periphery travel toward the wires and thus, are attentated by the resistive material of the electrodes. All the signals sum at the wires. As a result, the signals generated by the axial region of the transducing element are accentuated and those from the periphery are attenuated. This causes the echoes originating from the region of the main lobe to be weighed more heavily than echoes originating from other regions. Thus, the resistive electrodes also act to attenuate the side lobes upon reception.

The electrodes should be thin so they will not interfere with the mechanical waves which are transmitted or received (echoes). For good results, the thickness of the electrodes should be less than the wavelength of the mechanical waves in the external medium.

The resistance of the electrodes is a function of the resistivity of the resistive material, and the thickness of the electrodes. The intensity pattern of main and side lobes is a function of the resistance of the electrodes, since the resistance of the electrodes determines the attenuation of the mechanical waves which are transmitted and received by the transducing element. The Bessel function can be used to calculate this intensity pattern. Since there is a trade-off between the tolerance for sidelobes and the desire for high resolution of the main lobe, and the needs of each imaging system vary, there is no one ideal electrical resistance for the electrodes which can be stated now.

The second embodiment is similar to the first except that the second embodiment has only one resistive electrode. The other electrode is standard and highly conductive.

The third embodiment comprises standard, highly conductive electrode 20 and 21, such as gold or silver, around the axis of the electrode, and resistive, "washer-shaped" electrode 22 and 24 situated around the axial region. This washer-shaped region comprises the "peripheral" region of the electrode. In this patent application, "peripheral" refers to that region of the electrode or transducing element located outside of the axial region.

In the third embodiment, the peripheral electrodes connects to the outer edge of the highly conducting or "axial" electrode. FIGS. 4 and 5 show both the axial and peripheral electrodes of the third embodiment. This transducer transmits mechanical waves 24 with uniform intensity from its axial region, and mechanical waves 26 with lower, "tapered" intensity from its peripheral region. As a result of its axial, conductive electrode, the third embodiment produces mechanical waves which have a more intense and narrow main lobe than that of the transducer of the first embodiment (at any one frequency).

A Gausian distribution of the magnitude of the mechanical waves versus the displacement from the axis will produce little sidelobes. A high and constant level of energy transmitted from the axial region, and decreasing energy transmitted from the peripheral region may better approximate a Gausian distribution than the distribution of the first embodiment--peak energy at the axis only, and decreasing energy from regions located further and further away from the axis.

The fourth embodiment is similar to the third except only one electrode of the fourth embodiment has the axial and peripheral electrodes. The other electrode is a standard, highly conducting electrode.

The fifth embodiment of the invention is similar to the third except only one electrode of the fifth embodiment has the axial and peripheral electrodes. The other electrode is a single, resistive electrode.

The alternate embodiments of the invention comprise transducing elements and electrodes with similar electrical properties to those of the transducers previously described, but the shape of the alternate embodiments is different.